"However, if you instructed them to stop and back off and they didn't, then I think you would have reasonable grounds for defense."

Speaking from officer standpoint....If someone makes an attempt to take a weapon that justifies use of deadly force. If you do disengage without firing for whatever reason, like weapon out of battery/loose mag etc, then it is taught to back and off and tap/rack, bang OR then give commands to get on the ground or whatever. If they then move toward you again it is justifiable to shoot at that point as well as they have already demonstrated the intent to disarm/use deadly force and were just fortunate they were not shot the first time. That was taught in the last class I had on tactics and retention.